1. Field of the Invention
The present invention relates to a heat-assisted magnetic recording head for use in heat-assisted magnetic recording where a magnetic recording medium is irradiated with near-field light to lower the coercivity of the magnetic recording medium for data recording, and to a head gimbal assembly and a magnetic recording device each of which includes the heat-assisted magnetic recording head.
2. Description of the Related Art
Recently, magnetic recording devices such as a magnetic disk drive have been improved in recording density, and thin-film magnetic heads and magnetic recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a reproducing head including a magnetoresistive element (hereinafter, also referred to as MR element) intended for reading and a recording head including an induction-type electromagnetic transducer intended for writing are stacked on a substrate. Examples of the MR element include a giant magnetoresistive (GMR) element and a tunneling magnetoresistive (TMR) element. The recording head has a coil and a magnetic pole. The coil produces a magnetic field corresponding to data to be recorded on the magnetic recording medium. The magnetic pole allows a magnetic flux corresponding to the magnetic field produced by the coil to pass, and produces a recording magnetic field for recording data on the magnetic recording medium. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
Magnetic recording media are discrete media each made of an aggregate of magnetic fine particles, each magnetic fine particle forming a single-domain structure. A single recording bit of a magnetic recording medium is composed of a plurality of magnetic fine particles. For improved recording density, it is necessary to reduce asperities at the borders between adjoining recording bits. To achieve this, the magnetic fine particles must be made smaller. However, making the magnetic fine particles smaller causes the problem that the thermal stability of magnetization of the magnetic fine particles decreases with decreasing volume of the magnetic fine particles. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the magnetic recording medium, and this makes it difficult to perform data recording with existing magnetic heads.
To solve the foregoing problems, there has been proposed a technique so-called heat-assisted magnetic recording. This technique uses a magnetic recording medium having high coercivity. When recording data, a magnetic field and heat are simultaneously applied to the area of the magnetic recording medium where to record data, so that the area rises in temperature and drops in coercivity for data recording. Hereinafter, a magnetic head for use in heat-assisted magnetic recording will be referred to as a heat-assisted magnetic recording head.
In heat-assisted magnetic recording, near-field light is typically used as a means for applying heat to the magnetic recording medium. A commonly known method for generating near-field light is to use a near-field optical probe or so-called plasmon antenna, which is a piece of metal that generates near-field light from plasmons excited by irradiation with light.
In general, laser light that is used for generating near-field light is guided through a waveguide that is provided in the slider to the plasmon antenna that is located near the medium facing surface of the slider. Possible techniques of placement of a light source that emits the laser light are broadly classified into the following two. A first technique is to place the light source away from the slider. A second technique is to fix the light source to the slider.
The first technique is described in JP 2007-200475 A, for example. The second technique is described in U.S. Patent Application Publication No. 2008/0002298 A1 and U.S. Patent Application Publication No. 2008/0043360 A1, for example.
The first technique requires an optical path of extended length including such optical elements as a mirror, lens, and optical fiber in order to guide the light from the light source to the waveguide. This causes the problem of increasing energy loss of the light in the path. The second technique is free from the foregoing problem since the optical path for guiding the light from the light source to the waveguide is short.
The second technique, however, has the following problem. Hereinafter, the problem that can occur with the second technique will be described in detail. The second technique typically uses a laser diode as the light source. The laser diodes available include edge-emitting laser diodes and surface-emitting laser diodes. In an edge-emitting laser diode, the emission part for emitting the laser light is located in an end face that lies at an end of the laser diode in a direction parallel to the plane of an active layer. The emission part emits the laser light in the direction parallel to the plane of the active layer. In a surface-emitting laser diode, the emission part for emitting the laser light is located in a surface that lies at an end of the laser diode in a direction perpendicular to the plane of the active layer. The emission part emits the laser light in the direction perpendicular to the plane of the active layer.
The laser light emitted from a laser diode can be made incident on the waveguide by a technique described in U.S. Patent Application Publication No. 2008/0002298 A1, for example. This publication describes arranging a surface-emitting laser diode with its emission part opposed to the surface of the slider on the trailing side so that the laser light emitted from the emission part is incident on the waveguide from above. Surface-emitting laser diodes, however, typically have a lower optical output as compared with edge-emitting laser diodes. The technique therefore has the problem that it is difficult to provide laser light of sufficiently high intensity for use in generating near-field light.
The laser light emitted from a laser diode may be made incident on the waveguide by other techniques. For example, U.S. Patent Application Publication No. 2008/0043360 A1 describes a technique in which the incident end face of the waveguide is arranged at the surface opposite to the medium facing surface of the slider, and the laser diode is arranged with its emission part opposed to this incident end face so that the laser light emitted from the emission part is incident on the incident end face of the waveguide without the intervention of any optical element. This technique allows the use of an edge-emitting laser diode which has a high optical output. However, this technique has the problem that it is difficult to align the emission part of the laser diode with respect to the incident end face of the waveguide with high precision, since the position of the emission part of the laser diode can vary within a plane perpendicular to the optical axis of the waveguide.
To cope with this, the edge-emitting laser diode may be fixed to the top surface of the slider that lies at an end of the slider above the top surface of the substrate, so that the laser light is emitted in a direction parallel to the top surface of the slider, while arranging the waveguide so that the incident end face of the waveguide is opposed to the emission part of the laser diode. This configuration, however, has been found to have the following problem.
The laser diode generates heat during operation. When the MR element used in the reproducing head is subjected to heat, there occurs the problem that the MR element varies in resistance to suffer degradation in characteristics, and becomes breakable to suffer a decrease in life. In particular, a TMR element can easily suffer a dielectric breakdown across its thin tunnel barrier layer when subjected to heat. If an edge-emitting laser diode is fixed to the top surface of the slider as described above, much of the heat generated by the laser diode is transferred to the substrate of large volume and then transferred from the substrate to the magnetic recording medium. Much of the heat generated by the laser diode is thus released to the outside of the slider. Here, the part of the slider lying over the top surface of the substrate will be referred to as a head unit. The head unit includes the reproducing head and the recording head. Suppose that the laser diode is arranged so as not to overlap the MR element as viewed from above the top surface of the slider. In this case, the heat generated by the laser diode will not reach the MR element unless the heat spreads in the head unit in directions parallel to the top surface of the substrate. Actually, however, the heat generated by the laser diode spreads in the head unit in directions parallel to the top surface of the substrate, so that the heat reaches the MR element. This results in the above-mentioned problem associated with heat.
Moreover, the following two problems occur if two reproduction wiring layers, which are intended for supplying the MR element with a sense current for detecting a magnetic signal, are interposed at least in part between the bottom surface of the laser diode and the top surface of the substrate. The first problem is that the heat generated by the laser diode is transferred to the MR element through the reproduction wiring layers, thereby causing the above-mentioned problem associated with heat. The second problem is that a stray capacitance arises between the reproduction wiring layers and a conductive layer that is connected to an electrode that constitutes the bottom surface of the laser diode. The stray capacitance degrades the characteristics of the reproduction signal obtained by the MR element. In particular, if the MR element is a TMR element, which has a high resistance of 500Ω or above, the resistance component of the TMR element and the stray capacitance on the order of several picofarads combine to form a low-pass filter. This consequently degrades the high-frequency characteristics of the reproduction signal.